Before starting UMD, he used ourumd.com (which unfortunately seems not to have been updated for a few years now, so while the reviews may be “old” and some newer profs are not on there, it’s still pretty useful). However, after starting, he primarily went by word of mouth from other students, at least for engineering classes. One thing I remember him saying was that for some engineering classes, “TBA” (to be announced) for prof in a particular section was actually a chance worth signing up for. They would deliberately not name the prof until much later if a particular prof was really popular. That way, students were more likely to sign up for other sections and avoids overcrowding the waitlists (you cannot waitlist one section if you accept a seat in another…)